1. Field of the Invention
The present invention relates to a metallic gasket which is interposed between joining surfaces of a cylinder block and a cylinder head of an internal combustion engine, and which prevents leakage of fluid such as combustion gas, cooling water, lubricating oil, and the like.
2. Description of the Prior Art
Generally, a gasket of various kinds is interposed between joining surfaces of a cylinder block and a cylinder head constituting an automobile engine such as an internal combustion engine to prevent leakage of fluid such as combustion gas, cooling water, lubricating oil, and the like, and the sealing function is achieved by tightening both the cylinder block and the cylinder head by tightening members such as bolts or the like. In the prior art, as such a gasket, the so-called soft gasket is provided which is formed of an asbestos material, a chemical fiber material, a carbon sheet material, or the like. However, the soft gasket involves a drawback as an unavoidable problem due to its material that there is a deterioration with time due to the operation of the internal combustion engine, and that it is difficult to ensure a heat resistance. As a result, it is well known that recently, metallic gaskets are widely used, which are excellent in heat resistance, compressive resistance, and durability, and which provide satisfactory restoration properties (spring characteristics) and thermal conductivity.
Here, recently, in order to achieve further high performance of the engine, the developments have been progressed to make the engine small in size and light in weight, to generate high output, to reduce fuel consumption, and the like. For example, to achieve these objectives in the developments, there is a trend in which an aluminum alloy is used for components of the engine, and an interval between adjacent cylinder bores in the cylinder block is made as small as possible.
On the other hand, when the engine components are manufactured by using the aluminum alloy, as easily presumed, the overall rigidity is significantly reduced as compared with the case in which, for example, the engine components are manufactured by using cast iron. Furthermore, as a new drawback caused by making the engine small in size and light in weight, there is an increase in the amount of heat retained by the engine, which heat is generated due to the operation of the engine. To cope with this increase in the amount of heat retained by the engine, a cooling water path is expanded, or the like, however, the overall rigidity of the engine is further decreased. Furthermore, the cooling water path is formed in the vicinity of the cylinder bores to improve the cooling efficiently, however, due to this, the tightening positions of bolts or the like which tighten the cylinder block and cylinder head are located at positions remote from the cylinder bores. Thus, naturally, it is inevitable that the tightening force is large at positions near the bolts, and conversely, the tightening force is small at positions remote from the bolts.
When the overall rigidity of the engine is reduced, and a deviation in the tightening force by tightening members such as the bolts is increased, in a metallic gasket interposed between joining surfaces of the cylinder block and cylinder head, a gap is produced between the metallic gasket and the cylinder block, or between the metallic gasket and the cylinder head at circumferential edge portions of a cylinder bore hole formed corresponding to the cylinder bore. When such a phenomenon occurs, since a surface pressure is unbalanced at a peripheral portion of the cylinder bore hole, the roundness of the cylinder bore hole is degraded, and in addition, the deformation of the cylinder is increased due to differences in the thermal conductance at various portions from the cylinder block to the cylinder head during operation of the engine. Furthermore, when the above-mentioned gap is produced, since the joining surface side of the metallic gasket is directly exposed to combustion gas, drawbacks are caused in which the life of the metallic gasket is reduced, and it becomes impossible to ensure the sealing property for a long time.
For example, as shown in FIG. 20, in a metallic gasket 1 known in the art, a base plate 2 made of metal has a cylinder bore hole 3 formed therein, and a ring 4 obtained by forming a wire material in an annular shape is placed along an inner peripheral edge of the cylinder bore hole 3, and the ring 4 is fixed at its position by holding the ring 4 by a folded inner elastic metallic plate 5 which sandwiches an edge portion of the base plate 2, and furthermore, the inner elastic metallic plate 5 is covered and sandwiched by an outer elastic metallic plate 6. On the other hand, an outer peripheral end portion of the base plate 2 is sandwiched by another elastic metallic plate 7 different from the inner and outer elastic metallic plates 5 and 6. The elastic metallic plate 7 is welded to the base plate 2, for example, by spot welding. Moreover, the base plate 2 is formed with rubber beads 8 of silicon or the like around bolt holes, oil holes, water holes, and the like, and along the outer periphery of the base plate 2.
In such a metallic gasket 1, to cope with the above-mentioned reduction of the overall rigidity of the engine and the deviation in the tightening forces by the tightening members such as bolts or the like, the amount of collapse of the ring 4 and the amount of deformation of the rubber beads 8 are made different from each other, and at the same time, the amount of collapse of the ring 4 is restricted by selecting the thicknesses of the inner and outer elastic metallic plates 5 and 6, so as to ensure effective sealing at any position of the metallic gasket 1, and the tightening forces are made uniform by the elastic metallic plate 7 at the outer peripheral portion of the base plate 2 to suppress the overall deformation of the metallic gasket 1.
However, although it is true that the metallic gasket 1 as shown in FIG. 20 is expected to ensure sufficient sealing and to suppress the deformation, the base plate 2 and each of the elastic metallic plates 5, 6 and 7 are required to be formed as separate members, and also, at the time of manufacturing, the mounting of each of the elastic metallic plates 5, 6 and 7 to the base plate 2 must be made separately. As a result, the cost is inevitably increased, and the metallic gasket 1 becomes expensive. Moreover, in the metallic gasket 1, since only the metallic members such as the ring 4, inner and outer elastic metallic plates 5, and 6 are disposed around the cylinder bore, it is impossible to relieve the reduction of the overall rigidity of the engine and to absorb the deviation in the tightening forces of the bolts. Accordingly, there is a drawback that it becomes difficult to ensure the roundness of the cylinder bore.
Furthermore, in a metallic gasket shown in Japanese Patent Laid-Open Publication No. Hei 6-101761, in order to ensure high gas sealing, a grommet is mounted on a peripheral portion of a bolt hole, or the peripheral portion (a seal portion of the bolt hole) is made thicker than the other portions by folding back a portion of a base plate corresponding to the peripheral portion, and at the same time, the grommet is also mounted on a peripheral portion of a cylinder bore hole, or a portion of the base plate corresponding to the peripheral portion is folded back, and a restoring elastic member is inserted between the grommet and the base plate or between the folded portion and the base plate (a seal portion of the cylinder bore hole). Furthermore, a rubber baked seal line portion is formed between the bolt hole seal portion and the cylinder bore hole seal portion to seal water and oil.
However, in this case, at the time of tightening the bolts, for the inside area of the positions of the bolt holes, it is necessary to apply a load to deform the rubber baked seal line portion and a load to deform the restoring elastic member of the cylinder hole seal portion by the cylinder head. However, the rigidity of the cylinder head formed of an aluminum alloy required for a small and light engine is low, and a sufficient load is not applied. As a result, a portion between bolt holes of the cylinder head is deflected to form a convex surface curving upwardly. In particular, a large deflection is produced at a portion adjacent to the cylinder bore.
As mentioned above, when the large deflection is produced at the portion between the bolt holes adjacent to the cylinder bore of the cylinder head, it is difficult to seal a high temperature, high pressure combustion gas which is generated during operation of the engine. As a result, in the prior art metallic gaskets, such countermeasures have been adopted in which the thickness of the restoring elastic member at the cylinder bore hole seal portion is made larger as a whole, and a shim plate is partially inserted between adjacent cylinder bore holes to increase a surface pressure around the cylinder bore hole.
However, in these countermeasures, since the deformation of the cylinder head, that is, the deflection at the portion between the bolt holes is increased more and more, the surface pressure in the vicinity of the bolt hole is high due to the tightening of the bolts, and in contrast, the surface pressure at the portion between the bolt holes, in particular, at the portion adjacent to the cylinder bore becomes low. As a result, the cylinder inner tube surface is pushed inwardly by the portion having the strong surface pressure, that is, the portion in the vicinity of the hole hole, and when the bolt hole is formed at each of four corners around the cylinder bore hole, the cross-section of the cylinder inner tube is deformed in a clover shape and the roundness is lost. When the engine is operated in this state, an increase in oil consumption, loss of power, exhaust gas contamination, and the like will be caused.